Assisted reproductive technology (ART) includes such techniques as in vitro fertilization (IVF), artificial insemination (AI), intracytoplasmic sperm injection (ICSI) (other techniques using enucleated cells) and multiple ovulation and embryo transfer (MOET) (as well as other embryo transfer techniques), is used across the animal kingdom, including humans and other animals. ART methods are usually expensive, time consuming and marginally successful given the inherent fragility of gametes and embryos when outside of their natural environments. Furthermore, the use of ART within the animal breeding industry in a commercially feasible manner is additionally challenging due to the limited availability of genetically desirable gametes and zygotes. One way to lower the cost of ART and to improve its commercial feasibility is to increase the efficiency of the involved processes by improving the viability and overall quality of gametes and zygotes. Although there is has been a growing interest in this field over the course of the last decade or so, there still remains a strong need to increase the overall quality of gametes and zygotes for use in ART, especially when breeding focuses on pre-natal gender selection, including improving their viability (in the case of gametes and zygotes), their motility and fertility (in the case of sperm cells), as well as other longevity characteristics.
For example, in conventional AI, one problem limiting its commercial application in certain species is the need to use extremely high number of sperm cells per AI dose to ensure successful fertilization. Similarly, in IVF, the percentage of zygotes that develop into embryos remains frustratingly low; this high rate of loss significantly increases the cost of embryos and related services to end-users. There also remains the need for more efficient and lower cost procedures for improving post-embryo handling through cryopreservation as well as non-frozen transport. Cryopreservation of embryos is limited by the success rate of embryo production as well as blastocyst growth in vitro. Currently, only a marginal percentage of IVF embryos are suitable for cryopreservation which adds to the ongoing high cost of ART procedures.
Especially when processing gametes such as flushed oocytes or sperm cells, both conventional and sex-sorted, before their use in ART adds a tremendous amount of stress on the gamete cell and negatively impacts their cellular integrity and membrane structure which in turn is reflected in decreased viability, motility and fertility. An example of processing gametes prior to their use in ART is the sorting of sperm cells based on sex (known as “gender enrichment” or “sex-sorting”), which is a highly desired procedure to minimize wasted births of the wrong sex for selective breeding in the livestock industry but is often cost prohibitive and can be risky to those with smaller breeding herds.
The popular flow cytometry based sex-sorting process severely stresses and damages the cells and produces a low percentage of useful sperm, which although capable of fertilizing matured oocytes, have reduced viability, motility and fertility after the sex-sorting process. Typically, sex-sorting involves many harsh steps including but not limited to: the initial collection and handling of sperm ejaculate which naturally starts to deteriorate rapidly upon collection; the staining of sperm cells which involves binding of an excitable dye to the DNA or a harmful membrane selection procedure; the physical sorting of the sperm cells using high energy fluorescence that physically energizes the dye that is bound to the DNA, forced orientation through a narrow orifice, and application of an electrical charge to the cell; the physical collection of the cells into a container which often shocks the fragile cell upon contact; the osmotic stresses associated with dilution of the sperm droplet in collection media; and the storage of the sorted sperm usually by freezing which is well known to raise havoc with the cell's membrane systems. Each step places the processed sperm under abnormal stress which diminishes the overall motility, viability and/or fertility of the sperm. The result can lead to less efficient samples for use in ART, such as IVF and AI, and other types of subsequent or further processing.
Even non-sorted processed sperm exhibits significant losses in fertility, viability and motility when being collected, handled and transported without freezing, and noticeably experiences significant stress when mixed with cryoprotectant and is frozen and thawed. Many in the field have tried to improve methods for the use on unsorted, conventional semen to minimize loss in the handling processes associated with in vitro handling, preservation and use of semen samples.
Regardless of the processing, sperm lose their potential to fertilize when exposed to: elevated temperatures, abnormal buffers, stains, altered pH systems, physical pressurized orientation as when forced through a nozzle or when oscillated to form drops in a flow cytometer, radiation used to illuminate the DNA binding dye, physical stressors associated with separation and collection techniques, cryoprotectants, freezing, thawing and micromanipulation by the handler.
The large class of compounds referred to as antioxidants have been associated with providing beneficial effects to all sorts of cells, in vivo and in vitro, but these effects are as varied as the nature of the antioxidant itself. An antioxidant is simply one of a large variety of molecules that either inhibit the oxidation of another molecule, becomes oxidized itself in place of the target substrate, or binds harmful free radical intermediates and interrupts oxidative chain reactions within a cell. Most have dual roles; some are enzymes, others are non-enzymatic; some others are vitamins and others are cofactors. Such diversity lauds the diversity of antioxidants, but because of their known ability to minimize cell damage, they are frequently lumped together as a single class of compounds having only a single function, to bind free radicals.
Various antioxidants have shown promise in promoting cell integrity with some reports showing positive effects on sperm motility and membrane integrity during cryopreservation, but some tests have been shown to have minimal or even harmful effects on processed sperm.
Similarly, vitamins are again a rather diverse group of molecules having very different biological properties. Vitamins are any of a large group of organic compounds required in very small amounts as vital nutrients for an organism that cannot synthesize it. They can be antioxidants, enzymes, hormones or non-enzymes; they can be regulators of cell growth, cell differentiation or moderators of mineral metabolism.
To date, no studies have sufficiently addressed the use of antioxidants, vitamins or other supplements in the routine handling of fragile gametes during in vitro processing, especially during the harsh processing associated with the sex-sorting of sperm, whereby the end result is a reproducible improvement on the viability, motility and fertility of extensively processed sperm cells and embryos. There remains a continuing need to improve current methods of ART to reduce the cost and to make the procedures more dependable and commercially feasible to those on a tight budget, especially those smaller breeders who view sex-selection breeding as a high risk and expensive option.